Using Digital Sensors to Leverage Chips' Security

One way for an attacker to break a system is to perturb it. Expected effects are countermeasure deactivation or data corruption to disclose sensitive information. The prevention of such actions relies on detection of abnormal operating conditions. Digital sensors can play this role. A digital sensor is built out of the very same standard cells as the user logic to be protected. This ensures the advantage that the sensor and the user logic are exposed to the same stress. Balancing True positives and False negatives is a tough question in field of sensors. This is a general issue, and the best way to mitigate this paradox is to thoroughly investigate their properties, through simulations and real experiments. This results in characterizations, which in turn allows for intuitions on how to handle sensing values. In this paper, we exhibit the complex relationships between propagation times in logic and environmental conditions. Those results reinforce the relevance of the digital sensor versus the adversarial manipulation of environmental conditions: fewer false alarms are raised even if temperature (resp. voltage) is extreme, provided the effect is balanced by voltage (resp. temperature). Owing to the complex relationship between propagation delays, temperature and voltage, this cannot happen with a set of independent temperature and voltage sensors.